The invention relates to an apparatus, such as a pendulum, for damping or even eliminating low excitation frequencies of a structural component such as a helicopter rotor blade or the like.
In connection with helicopters vibrations in the helicopter body or nacelle are primarily caused by periodically occurring forces and moments which are transmitted by the helicopter rotor blades into the helicopter body or nacelle. Due to the transmission of these forces and moments, also referred to as loads, from the rotating system, including the helicopter rotor, into the fixed or stationary system, including the helicopter body, only certain so-called rotor harmonic forces and moments may be transmitted and these forces and moments are dependent on the number of the rotor blades. In the instance of a hingeless helicopter rotor having four rotor blades and due to the just stated fact, the so-called 3.OMEGA. and 5.OMEGA. moments at the rotor blade foot or neck are controlling for the low frequency nacelle vibrations. In this connection .OMEGA. designates the so-called rated rotary frequency which, for example, in the mentioned helicopter type is about 7 Hertz corresponding to 44.4 radians per second (rad/s). The vibration suppressor, in the example here described a governor or centrifugal pendulum, must be constructed with due regard to the frequency to be suppressed in this instance, for example 3.OMEGA. equal to 21 Hz. Such construction is necessary so that the vibration suppressor is capable of absorbing the periodic forces or moments which are otherwise capable of being transmitted from the helicopter rotor blades to the helicopter nacelle.
It would also be advantageous to further reduce the vibrations in the helicopter nacelle which are caused by the next higher harmonic forces in the present instance, for example, the 5.OMEGA. forces and moments. For this purpose a further vibration suppressor would have to be installed, for example, in a 5.OMEGA. governor or centrifugal pendulum. However, such installation is not possible or only possible with great difficulties due to the limited space available for such purpose.